Heat-assisted magnetic recording apparatus that modulates laser power to reduce differences between track widths of recorded marks

ABSTRACT

Two or more different elapsed time values are determined between transitions of a data signal applied to a magnetic write transducer of a heat-assisted magnetic recording apparatus. Two or more different power values of the laser are respectively associated with the two or more different elapsed time values. The two or more different power levels are selected to reduce differences between track widths of recorded marks having the two or more different elapsed time values.

SUMMARY

The present disclosure is directed to modulating a laser power signalduring heat-assisted magnetic recording. In one embodiment, two or moredifferent elapsed time values are determined between transitions of adata signal applied to a magnetic write transducer of a heat-assistedmagnetic recording apparatus. The magnetic write transducer applies amagnetic field to a recording medium in response to the data signal. Alaser power signal is applied to a laser that heats the recording mediumwhile the magnetic field is applied. Two or more different power valuesof the laser are respectively associated with the two or more differentelapsed time values. The two or more different power levels are selectedto reduce differences between track widths of recorded marks having thetwo or more different elapsed time values. Before the data signal isbetween two operational transitions, an operational elapsed time betweenthe two operational transitions is determined. When the data signal isbetween the two operational transitions, the laser power signal is setat a selected one of the two or more different power values in responseto the operational elapsed time corresponding to one of the two or moredifferent values of the elapsed times.

In another embodiment, transitions of a data signal applied to amagnetic write transducer of a heat-assisted magnetic recordingapparatus are determined. The magnetic write transducer applies amagnetic field to a recording medium in response to the data signal. Alaser power signal is applied to a laser that heats the recording mediumwhile the magnetic field is applied. Overshoot pulses to the laser powersignal. Each overshoot pulse corresponds one of the transitions of thedata signal. The overshoot pulses have magnitude and duration thatreduce differences between track widths of recorded marks having two ormore different elapsed time values between the transitions. These andother features and aspects of various embodiments may be understood inview of the following detailed discussion and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The discussion below makes reference to the following figures, whereinthe same reference number may be used to identify the similar/samecomponent in multiple figures.

FIG. 1 is a perspective view of a slider assembly according to anexample embodiment;

FIG. 2 is a graph showing track width for various clock timings used inrecording data according to an example embodiment;

FIG. 3 is a graph of dibit response in a data storage device accordingto an example embodiment;

FIG. 4 is a graph illustrating the determination of writer-to-laserdelay in a data storage device according to an example embodiment;

FIG. 5A is a diagram of a recording medium according to an exampleembodiment and FIG. 5B is a graph showing a writer data signal and amodulated laser power signal according to an example embodiment;

FIGS. 6A and 6B are a schematic diagrams of laser waveform generatorsaccording to example embodiments;

FIG. 7-10 are signal diagrams illustrating laser modulation schemesaccording to example embodiments;

FIG. 11 is a diagram of an apparatus according to an example embodiment;and

FIGS. 12-13 are flowcharts of methods according to example embodiments.

DETAILED DESCRIPTION

The present disclosure generally relates to data storage devices thatutilize magnetic storage media, e.g., magnetic disks. For example, ahard disk drive (HDD) unit contains one or more magnetic disks that arewritten to and read from using a magnetic read/write head attached tothe end of an arm that is positioned over tracks in the disk. To recorddata, the read/write head generates magnetic fields using a magneticcoil, the fields being directed to the magnetic disk surface via a writepole. To read data, the read/write head senses changes in magnetic fieldvia a sensor such as a magneto-resistive stack that is held proximate tothe moving disk. A disk drive typically has multiple heads, one for eachdisk surface.

In order to increase ADC in magnetic storage, some drives utilize atechnology known as heat-assisted magnetic recording (HAMR). In FIG. 1,a perspective view shows elements of a HAMR read/write head 100according to an example embodiment. The read/write head 100 may bereferred to herein as a recording head, write head, read head, slider,etc. Generally, a HAMR read/write head 100 includes a heat source (e.g.,laser diode 106) that directs energy to a magnetic disk (not shown) viaoptical components (e.g., waveguide 110) integrated into the recordinghead. The energy creates a hotspot on the disk, lowering its magneticcoercivity and allowing a write pole (which is part of read/writetransducer 108) to set magnetic orientation at the hotspot. Because ofthe small size of the hotspot, this allows for recording smaller bitsthan is currently possible with conventional perpendicular magneticrecording (PMR).

This disclosure describes a laser power modulation scheme for HAMRdevices in which the laser power is modulated based on the pattern ofbits recorded by the magnetic writer. This allows, for example, makingthe track width substantially equal for marks of all lengths. Thisimproves the quality of the recorded short marks, and will enable gainsin either linear density, track density, or both.

In conventional HAMR, the laser power remains constant for the durationof a sector. The result is that short marks (e.g., 1T and 2T) arenarrower in the cross track direction than longer marks (e.g., ≥3T),which degrades the signal-to-noise ratio (SNR) associated with the bitsrecorded in shorter marks. In FIG. 2, a graph illustrates this byshowing track scans for various frequencies. The track scans for the 1Tand 2T tones are significantly narrower than the lower frequencycounterparts (e.g., 3T-8T). Alternatively, if the laser power isselected such that the short marks have adequate SNR, then the longmarks are wider than necessary, which degrades the tracks per inchcapability (TPIC). In embodiments described below, apparatuses andmethods can ensure the track width and recording quality of all bits aresubstantially the same regardless of mark length.

The track width in HAMR is known to vary with the laser power usedduring recording. Consequently, the laser power can be adjusted toreduce track width differences and/or SNR differences for different marklengths. This can be achieved by modulating the laser coherently withthe magnetic writer data, e.g., with a pattern based on the mark lengthsof the data being recorded by the magnetic writer. Laser modulation canbe accomplished by including in the preamp a laser modulation waveformgenerator that is added to the nominal laser current. The modulationwaveform can be generated by looking at the upcoming bits about to bewritten to determine the mark length and then adjusting the lasercurrent accordingly. There are several possible embodiments for lasermodulation, which are described in more detail below.

In implementing this proposed solution, several factors are consideredto ensure the data is recorded accurately. First, since changing thelaser power moves the recording location in the down track direction, anadditional calculation can be done to cancel this shift. The timingshift required for this correction, which is added onto any existingform of precompensation, is given by δt=((T_(w)−T_(a))(δP/P))/(vdT/dx),where T_(w) is the write temperature, T_(a) is the ambient temperature,δP/P is the fractional change in laser power, v is the linear velocity,and dT/dx is the down track thermal gradient. The formula itself may bederived from that used to experimentally characterize the thermalgradient in HAMR. For example, see H. J. Richter et al., IEEE Trans.Magn. 49, 5378 (2013), D. A. Saunders et al., IEEE Trans. Magn. 53,3100305 (2017), I. Gilbert et al., IEEE Trans. Magn. 55, 3001006 (2019),and commonly-owned U.S. Pat. No. 10,339,963. Note that the timing shiftsproduced by the pattern-dependent variations in the laser power producespecific echoes in the dibit response, as shown by the arrows in FIG. 3.The dibit responses in FIG. 3 were extracted from pseudo-random bitsequences recorded with a variety of laser modulation schemes. Theseechoes can be characterized and used to verify the appropriate level oftiming compensation required.

In order to ensure that magnetic writer waveform and the laser currentwaveform can be accurately and precisely aligned in time, the preamp mayinclude a variable delay element in the circuitry producing thesewaveforms. One example method for measuring the correct writer-laserdelay is to sweep the delay and measure the resulting bit error rate(BER), as shown in FIG. 4, where bit error rate is plotted as a functionof time delay between the magnetic writer and the laser modulation.

In addition to the corrections for laser current change-induced shiftsand the timing delay described above, it may be beneficial to introducesmall additional delays to improve the fidelity of the timing of therecording or the quality of the transitions. For example, the individualmagnetic transitions may be delayed such that they always occur when thelaser current is elevated, which produces a higher down track thermalgradient and consequently better linear density, or bits per inchcapability (BPIC). In addition, because the magnetic recording media'stemperature may respond to variations in the laser current on timescales less than a single bit, it may be beneficial to deliberately tunethe media properties to allow for rapid thermal response.

One non-limiting example of a recording medium 520 tuned for rapidthermal response is shown in FIG. 5A. The magnetic recording medium 520includes a metal heatsink film 522. The heatsink film 522 may have acrystalline structure, such as body-centered-cubic (bcc) formed ofmaterials including but not limited to Cr, Mo, W and alloys thereof. Theheatsink film may have a face-centered-cubic structure including but notlimited to Cu, Ag, Au and alloys thereof. The thickness of the heatsinklayer 522 is matched to the dimensions of the near-field-transducer inthe recording head. The thickness can be adjusted so the product ofthermal conductivity and thickness gives the desired time response (andtotal laser power requirement). In one embodiment, the range of heatsinklayer thickness is 20-50 nm. Further details of recording mediaheatsinks may be found in commonly owned U.S. Pat. Nos. 8,765,273 and9,502,065.

The heatsink film 522 may be deposited on a seed layer 528 which coversa glass substrate 530. A magnetic recording layer 524 (e.g., CoPt) isdeposited on the alloy heatsink layer 522. The magnetic orientationwithin the recording layer 524 stores the bits recorded to the recordingmedium 520. A protective overcoat 526 covers the magnetic recordinglayer 524. The heat sink layer 522 should be of sufficient thickness toconduct all the heat deposited by the recording head's near fieldtransducer. Additionally, the heat sink layer 522 should be fabricatedfrom a material with the maximum possible thermal conductivity. Ideallythe insertion and tuning of the heat sink layer 522 should have minimalimpact on the thermal, optical, and magnetic properties of the othermaterials above it in the media stack.

An example of writer and laser data 500, 501 usable in a HAMR drive areshown in the diagram of FIG. 5B. Dotted line 502 represents a timereference from which both data 500, 501 may be synced as describedabove, e.g., at the beginning of a write sequence. In some embodimentsdescribed below, a HAMR drive determines at least two different elapsedtime values 504-506 between transitions of the write data 500 thatdrives a write transducer. The laser power signal 501 is applied to alaser that heats the recording medium while the changing magnetic fieldthat produces data 500 is applied by the write transducer.

Two or more different power laser values 508-510 are associated with thetwo or more different elapsed time values, e.g., to reduce differencesbetween track widths of recorded marks having the two or more differentelapsed time values 504-506. Note that for this and other figuresherein, the terms “marks” and “recorded marks” refer to data regionsrecorded to the medium at any period between two transitions 512-516 ofthe write data 500. In this example, the power levels 508-510 aremeasured relative to a level 511 that corresponds to the laser beingturned off or idle. This level 511 could correspond to zero current, orto a non-zero current close to a bias threshold of the laser, e.g., justbefore significant lasing occurs.

While the write data 500 is between transitions 512-516, the laser powersignal 501 to is set to one of the different power values 508-510 inresponse to the transitions being separated by the respective two ormore different values of the elapsed times. Generally, the power values508-510 are set to a highest power value for a shortest of the elapsedtimes 504-506 and to a lowest power value for a longest of the elapsedtimes 504-506. Intermediate values of power between the highest andlowest powers are associated with intermediate elapsed time between theshortest and longest times as appropriate, e.g., distributed linearly oraccording to some other function. Various ways of determining andsetting the power values 508-510 are described below, as are alternateembodiments where modulating the laser power signal does not rely ondetermining the elapsed times 504-506 between transitions 512-516.

The laser modulation and writer waveforms may be generated externally(e.g., in the lab) using a computer and/or an arbitrary waveformgenerator (AWG). For applications to HAMR drives, both waveforms may begenerated internally using circuitry incorporated into the preamp IC. Inthe drive, a source such as the system clock may be used to generate theappropriate waveform shown here being generated by AWG 600. In eithercase the circuits may include a high bandwidth (e.g., 7 GHz) laseramplifier to increase the amplitude of the laser modulation waveform tothe correct level.

In FIG. 6A, a schematic diagram shows a waveform generator circuitaccording to an example embodiment. A waveform generator 600 outputs twosignals 602, 604 for use in driving a laser via a preamplifier 606. In aHAMR drive, a source such as the system clock may be used to generatethe waveforms shown here being generated by waveform generator 600.Signal 602 is a laser modulation signal that is amplified by a highspeed laser driver 603. The amplified modulation signal 605 is used togenerate a laser input signal 616. The signal 604 provided from thewaveform generator 600 is used to generate a write signal 614 sent to awrite coil. A delay element 608 can adjustably delay the transitions ofthe signal 604 to ensure synchronization of the modulated laser outputsignal 616 with the write transducer signal. The preamplifier 606 may bea high-bandwidth amplifier as noted above.

In FIG. 6A, the write signal 604 is used to extract a base current 611that is combined 612 with the laser modulation signal 602 to produce thelaser input signal 616. In FIG. 6B, a schematic diagram shows a waveformgenerator circuit according to an example embodiment. In FIG. 6B, thewrite signal path is shown using the same reference numbers as in FIG.6A, although the individual components and signals may be configureddifferently. In FIG. 6B, the laser modulation output 602 is amplified toprovide the laser input 616 without any combination with the writersignal 604. In the embodiment of FIG. 6B, the waveguide generator 600will provide a more complex waveform shape that results in the laserinput 616 having the base current offsets 611 that were added via thewrite signal 604 in the embodiment of FIG. 6A.

The nominal laser power value as well as the pulse values may be foundin a number of different ways as described below. In FIGS. 7-10, writerdata and laser current signals are shown superimposed over the same timeline, showing the determination of laser power levels according toexample embodiments. Note that in these figures laser current is used torepresent a time-varying amount of power applied to the laser, howeverit will be understood that other signal values (e.g., voltage) may alsobe used to similarly represent the control of laser power withoutdeviating from the scope of these embodiments.

In FIG. 7, a diagram shows a modulated laser power waveform according toan example embodiment. In this embodiment, a nominal laser power 700 isselected such that with no modulation of the laser power, the longestbits/marks have the desired track width. The longest bits/markscorrespond to the longest elapsed time between adjacent two transitionsof the writer data signal. The laser power for the short marks (anon-restrictive example being the 1T and 2T marks) is increased (e.g.,as indicated, for example, by local high levels 701, 702) until thesemarks also have the desired track width.

Note that this modulation scheme takes all of its areal densitycapability (ADC) gains in BPIC. The level of laser power modulation willvary depending on the length of the short mark, e.g., 20% for the 1Tmarks and 10% for the 2T marks. The level of laser modulation may bedetermined on a finer time scale than this if precompensation is beingused. For example, for certain precompensation settings, the penultimatebit in the NRZ sequence 1101 may have a different length than that inthe sequence 0101. In this case, the level of laser modulation may bedetermined by the actual physical length of the mark in question ratherthan by the number of bits it encodes. Also note that overshoot pulses703 may be optionally applied to the laser power inputs, where eachovershoot pulse 703 corresponds to a transition of the writer datasignal. These pulses 703 are described in greater detail in thediscussion of FIG. 10 below.

In FIG. 8, a diagram shows a modulated laser power waveform according toan example embodiment. In this embodiment, a nominal laser power 800 isselected such that with no modulation of the laser power, the shortestmarks (e.g., the 1T marks) have the desired track width. Then the laserpower for the longer marks (a non-restrictive example being 2T and ≥3Tmarks) is decreased (e.g., as indicated, for example, by local low powerlevels 801, 802) until these marks also have the desired track width.Note that this modulation scheme takes all of its ADC gains in TPIC. Asin the embodiment in FIG. 7, the levels of laser modulation may be morefinely tuned based on changes to the bit length induced byprecompensation. Also note that overshoot pulses 803 may be optionallyapplied to the laser power inputs, where each pulse 803 corresponds to atransition of the writer data signal. These pulses 803 are described ingreater detail in the discussion of FIG. 10 below.

In FIG. 9, a diagram shows a modulated laser power waveform according toan example embodiment. In this embodiment, the nominal laser power 900is selected at an intermediate value between that used in embodimentsshown in FIGS. 7 and 8. This may be done by averaging the nominal laserpowers from embodiments shown in FIGS. 7 and 8 or some other method,e.g., optimizing for overall SNR or BER. Then the laser power isincreased (as indicated, for example, by local high power level 902) forthe short marks (e.g., 1T and 2T) and decreased (as indicated, forexample, by local low power level 901) for the long marks (e.g., ≥4T)until the SNR or BER is further optimized.

Note that this modulation scheme may take some of its ADC gain in BPICand some in TPIC. A As in the embodiment in FIG. 7, the levels of lasermodulation may be more finely tuned based on changes to the bit lengthinduced by precompensation. Another equivalent method to generate thelaser modulation waveforms for this case is to increase the laser powerfor the short marks only and not decrease the laser power for the longmarks, and then AC couple this waveform to the nominal laser current.Also note that overshoot pulses 903 may be optionally applied to thelaser power inputs, where each pulse 903 corresponds to a transition ofthe writer data signal. These pulses 903 are described in greater detailin the discussion of FIG. 10 below.

In FIG. 10, a diagram shows modulated laser power waveforms according toan example embodiment. In this embodiment, an overshoot pulse 1001 isapplied to the laser power every time a transition is passed. Thisovershoot pulse 1001 may be of constant amplitude and duration for everytransition, or it may vary based on the number of bits in the mark or onthe physical length of the mark (which also corresponds to elapsed timesbetween the write data signal). This overshoot may be applied torelative to constant nominal laser current 1000, or it may be applied ontop of the laser current modulations described in embodiments shown inFIGS. 7-9 (see pulses 703, 803, and 903 in FIGS. 7-9).

Because the laser current for the short marks (e.g., 1T and 2T marks) isequal to the nominal laser current plus the overshoot for most of thelength of the mark, whereas the laser current for the long marks (e.g.,≥3T marks) is equal to only the nominal laser current for most of thelength of the mark, the net effect is similar to the laser currentmodulation schemes of embodiments shown in FIGS. 7-9. The embodiment inFIG. 10 has an additional benefit, however, of equalizing the trackwidth for the entire length of a long mark, rather than having the firstfew bits narrow and the rest wide, as may be the case for theembodiments shown in FIGS. 7-9.

Because the laser current/power signal in FIG. 10 also varies both aboveand below the nominal value similar to the embodiment shown in FIG. 9,the nominal value 1000 may be determined similarly as described in theembodiment shown in FIG. 9. Note that in this case, the offset/magnitude1002 and duration 1003 of the overshoot may be taken into account whendetermining the nominal value 1000, e.g., by assuming an a prioridistribution of overshoot pulses as well as their magnitude 1002 andduration 1003. Alternatively, a nominal value 1000 may be first derived,and then the magnitude 1002 and duration 1003 may be adjusted until somecriteria is met, e.g., minimizing BER, dibit response, etc.

In FIG. 11, a block diagram illustrates a data storage apparatus 1100according to an example embodiment. Control logic circuit 1102 of theapparatus 1100 includes a system controller 1104 that processes read andwrite commands and associated data from a host device 1106. The hostdevice 1106 may include any electronic device that can becommunicatively coupled to store and retrieve data from a data storagedevice, e.g., a computer, peripheral card, etc. The system controller1104 is coupled to a read/write channel 1108 that reads from and writesto a surface of a magnetic disk 1110.

The read/write channel 1108 generally converts data between the digitalsignals processed by the controller 1104 and the analog signalsconducted through one or more read/write heads 1112 during readoperations. To facilitate the read operations, the read/write channel1108 may include analog and digital interface circuitry such aspreamplifiers, filters, decoders, digital-to-analog converters,timing-correction units, etc. The read/write channel 1108 also providesservo data read from servo wedges 1114 on the magnetic disk 1110 to aservo controller 1116. The servo controller 1116 uses these signals toprovide a voice coil motor control signal 1117 to a VCM 1118. The VCM1118 moves (e.g., rotates) an arm 1120 upon which the read/write heads1112 are mounted in response to the voice coil motor control signal1117.

Data within the servo wedges 1114 is used to detect the location of aread/write head 1112 relative to the magnetic disk 1110. The servocontroller 1116 uses servo data to move a read/write head 1112 to anaddressed track 1122 and block on the magnetic disk 1110 in response tothe read/write commands (seek mode). While data is being written toand/or read from the disk 1110, the servo data is also used to maintainthe read/write head 1112 aligned with the track 1122 (track followingmode).

The disk drive 1100 uses HAMR, and therefore the read/write heads 1112include an energy source (e.g., laser diode) that heats the magneticdisk 1110 when recording. A HAMR laser control block 1123 sends acurrent to activate the lasers when recording. To assist in detectingand compensating for variations in the application of heat to the disk,a write data monitor 1124 examines write data signals targeted for themagnetic writer on the read/write head 1112. The data monitored by thewrite monitor 1124 may at least include transitions of the write signalas a function of time, as well as elapsed time between subsequenttransitions in some embodiments. The write monitor 1124 sends data tothe HAMR laser control 1123 to modulate the laser power as described invarious embodiments herein.

In FIG. 12, a flowchart shows a method according to an exampleembodiment. The method involves determining 1200 two or more differentelapsed time values between transitions of a data signal applied to amagnetic write transducer of a heat-assisted magnetic recordingapparatus. The magnetic write transducer applies a magnetic field to arecording medium in response to the data signal. A laser power signal(e.g., current) is applied to a laser that heats the recording mediumwhile the magnetic field is applied. Note that for this method, thedetermining 1200 of the elapsed time values may involve reading valuesof elapsed time from a persistent memory.

Two or more different power values are associated 1201 with the two ormore different elapsed time values. The two or more different powerlevels are selected to reduce differences between track widths ofrecorded marks having the two or more different elapsed time values.Again, the association 1201 of the power levels with the elapsed timevalues may be determined via a structure in memory, e.g., a map betweenelapsed time and power values.

At block 1202, it is determined if writing is occurring. If so (block1202 returns ‘yes’), it is determined whether there are upcoming twooperational transitions of the data signal at block 1203. Note thatwhile recording a sequence of marks, block 1203 will always return ‘yes’until the last mark in the sequence is being written, after which thesystem will stop writing (block 1202 will return ‘no’). Before the datasignal is between the two operational transitions, an operationalelapsed time between the two operational transitions determined 1204.When the data signal is between the two operational transitions (block1205 returns ‘yes’), the laser power signal is set 1206 at a selectedone of the two or more different power values in response to theoperational elapsed time corresponding to one of the two or moredifferent values of the elapsed times.

In FIG. 13, a flowchart shows a method according to another exampleembodiment. The method involves determining 1300 determining magnitudeand duration of overshoot values added to a laser signal to reducedifferences between track widths of recorded marks having two or moredifferent elapsed time values between transitions. This determination1300 may be made by reading values from a persistent memory. Whilewriting (block 1301 returns ‘yes’), transitions of a data signal appliedto a magnetic write transducer of a heat-assisted magnetic recordingapparatus are determined 1302. The magnetic write transducer apply amagnetic field to a recording medium in response to the data signal, alaser power signal being applied to a laser that heats the recordingmedium while the magnetic field is applied. Overshoots pulses are added1303 to the laser power signal, each overshoot pulse corresponding oneof the transitions of the data signal. The overshoot pulses have themagnitude and duration determined at block 1300.

The foregoing description of the example embodiments has been presentedfor the purposes of illustration and description. It is not intended tobe exhaustive or to limit the embodiments to the precise form disclosed.Many modifications and variations are possible in light of the aboveteaching. Any or all features of the disclosed embodiments can beapplied individually or in any combination are not meant to be limiting,but purely illustrative. It is intended that the scope of the inventionbe limited not with this detailed description, but rather determined bythe claims appended hereto.

What is claimed is:
 1. A method, comprising: determining two or moredifferent elapsed time values between transitions of a data signalapplied to a magnetic write transducer of a heat-assisted magneticrecording apparatus, the magnetic write transducer applying a magneticfield to a recording medium in response to the data signal, a laserpower signal being applied to a laser that heats the recording mediumwhile the magnetic field is applied; respectively associating two ormore different power values of the laser with the two or more differentelapsed time values, the two or more different power levels selected toreduce differences between track widths of recorded marks having the twoor more different elapsed time values; before the data signal is betweentwo operational transitions, determining an operational elapsed timebetween the two operational transitions; and when the data signal isbetween the two operational transitions, setting the laser power signalat a selected one of the two or more different power values in responseto the operational elapsed time corresponding to one of the two or moredifferent values of the elapsed times.
 2. The method of claim 1, whereinthe laser power signal is offset from the data signal by a delay, thedelay selected by: sweeping the delay from a first value to a secondvalue while recording test data to the recording medium; reading biterror rate of the test data; and selecting a value of the delay thatresults in a minimum value of the bit error rate.
 3. The method of claim1, further comprising: defining a nominal power value of two or moredifferent power values such that recorded marks with a longest of thetwo or more elapsed time values have a target track width at the nominalpower value; and defining a second power value of the two or moredifferent power values such that recorded marks with a shortest of thetwo or more elapsed time values has the target track width at the secondpower value, the second power value being higher than the nominal powervalue.
 4. The method of claim 3, further comprising defining a thirdpower value of the two or more different power values that is betweenthe nominal power value and the second power value, the third valueassociated with an intermediate elapsed time between the shortest andthe longest elapsed times.
 5. The method of claim 1, further comprising:defining a nominal power value of two or more different power valuessuch that recorded marks with a shortest of the two or more elapsed timevalues have a target track width at the nominal power value; anddefining a second power value of the two or more different power valuessuch that recorded marks with a longest of the two or more elapsed timevalues have the target track width at the second power value, the secondpower value being lower than the nominal power value.
 6. The method ofclaim 1, further comprising: defining a nominal power value of two ormore different power values such that recorded marks with an elapsedtime between a longest and shortest of the two or more elapsed timevalues has a target track width at the nominal power value; and definingsecond and third power values of the two or more different power valuesrespectively associated with the longest and shortest of the two or moreelapsed time values such that recorded marks with the longest andshortest of the two or more elapsed time values have the target trackwidth at the second and third power values, the s second and third powervalues value being respectively lower and higher than the nominal powervalue.
 7. The method of claim 1, further comprising adding an overshootto the laser power signal for each of the transitions of a data signal.8. The method of claim 1, further comprising heat sinking a recordinglayer of the recording medium via a heat sink layer located below therecording layer, the heat sinking of the recording layer increasing athermal response of the recording medium.
 9. A heat-assisted magneticrecording apparatus comprising: interface circuitry operable tocommunicate with a magnetic write transducer and a laser, the laserheating a recording medium in response to a laser power signal while themagnetic field is applied to the recording medium; a controller coupledto the interface circuitry and configured to: determine two or moredifferent elapsed time values between transitions of a data signalapplied to the magnetic write transducer; respectively associate two ormore different power values of the laser with the two or more differentelapsed time values, the two or more different power levels selected toreduce differences between track widths of recorded marks having the twoor more different elapsed time values; before the data signal is betweentwo operational transitions, determine an operational elapsed timebetween the two operational transitions; and when the data signal isbetween the two operational transitions, set the laser power signal at aselected one of the two or more different power values in response tothe operational elapsed time corresponding to one of the two or moredifferent values of the elapsed times.
 10. The apparatus of claim 9,wherein the laser power signal is offset from the data signal by adelay, the delay selected by: sweeping the delay from a first value to asecond value while recording test data to the recording medium; readingbit error rate of the test data; and selecting a value of the delay thatresults in a minimum value of the bit error rate.
 11. The apparatus ofclaim 9, wherein the controller is further configured to: define anominal power value of two or more different power values such thatrecorded marks with a longest of the two or more elapsed time valueshave a target track width at the nominal power value; and define asecond power value of the two or more different power values such thatrecorded marks with a shortest of the two or more elapsed time valueshas the target track width at the second power value, the second powervalue being higher than the nominal power value.
 12. The apparatus ofclaim 11, wherein the controller is further configured to define a thirdpower value of the two or more different power values that is betweenthe nominal power value and the second power value, the third valueassociated with an intermediate elapsed time between the shortest andthe longest elapsed times.
 13. The apparatus of claim 9, wherein thecontroller is further configured to: define a nominal power value of twoor more different power values such that recorded marks with a shortestof the two or more elapsed time values have a target track width at thenominal power value; and define a second power value of the two or moredifferent power values such that recorded marks with a longest of thetwo or more elapsed time values have the target track width at thesecond power value, the second power value being lower than the nominalpower value.
 14. The apparatus of claim 9, wherein the controller isfurther configured to: define a nominal power value of two or moredifferent power values such that recorded marks with an elapsed timebetween a longest and shortest of the two or more elapsed time valueshas a target track width at the nominal power value; and define secondand third power values of the two or more different power valuesrespectively associated with the longest and shortest of the two or moreelapsed time values such that recorded marks with the longest andshortest of the two or more elapsed time values have the target trackwidth at the second and third power values, the s second and third powervalues value being respectively lower and higher than the nominal powervalue.
 15. The apparatus of claim 9, further comprising adding anovershoot to the laser power signal for each of the transitions of adata signal.
 16. The apparatus of claim 9, further comprising therecording medium, wherein a recording layer of the recording medium isformed on a heat sink layer, the heat sink layer increasing a thermalresponse of the recording medium.
 17. A method, comprising: determiningtransitions of a data signal applied to a magnetic write transducer of aheat-assisted magnetic recording apparatus, the magnetic writetransducer applying a magnetic field to a recording medium in responseto the data signal, a laser power signal being applied to a laser thatheats the recording medium while the magnetic field is applied; andadding overshoot pulses to the laser power signal, each overshoot pulsecorresponding one of the transitions of the data signal, the overshootpulses having magnitude and duration that reduce differences betweentrack widths of recorded marks having two or more different elapsed timevalues between the transitions.
 18. The method of claim 17, wherein thelaser power signal is offset from the data signal by a delay, the delayselected by: sweeping the delay from a first value to a second valuewhile recording test data to the recording medium; reading bit errorrate of the test data; and selecting a value of the delay that resultsin a minimum value of the bit error rate.
 19. The method of claim 17,wherein the magnitude and duration are the same for all of thetransitions.
 20. The method of claim 17, wherein the magnitude andduration vary for different transitions based on at least one of anumber of bits in the recorded mark or on a physical length of therecorded mark.